Resistive touchscreens generally comprise two sheets coated with a resistive material and separated by an airgap. The front sheet is made of a thin and flexible material, and the back sheet, usually glass, may be a thicker, less flexible material. The surfaces of both sheets on the airgap side may be coated with a transparent conductive material, such as indium tin oxide (ITO). Touching the front sheet with, for example, a finger or a stylus, causes it to deform so that it contacts the glass panel allowing the location of the touch to be determined by measurements of resistance.
In order to avoid damage to sensitive touchscreen components and components inside the enclosure by exposure to dirt, particles or corrosive vapors, it is important to provide a reliable front seal between the touchscreen and a protective front casing. In existing practice, the front seal is usually made by interposing a gasket between the front sheet and a bezel, with the bezel being that part of the front casing which overlaps the front sheet. Since the front sheet is necessarily thin and flexible, it has been difficult in existing practice to create a reliable front seal without the gasket deforming or restraining the front sheet in such a way that correct operation of the touchscreen is compromised or even prevented. The difficulty in sealing resistive touchscreens is to be contrasted with capacitive touchscreens in which a relatively thick protective glass front cover may be used, allowing much easier sealing to the front cover.
Use of a gasket on top of the front sheet is largely driven by the need to make electrical contacts available outside the touchscreen. In existing practice this is usually done by arranging for a suitable flat touchscreen cable to emerge from the plane of the airgap. To allow the cable to emerge behind the front cover, the front seal needs to be outside the plane of the airgap, namely on top of the front sheet. A similar consideration applies to the need for a venting aperture in the plane of the airgap. Use of a venting aperture is optional, but when required its purpose is to allow equalization of the airgap pressure with the external environment.
The need for a bezel to accomplish the front seal is also a significant disadvantage. Unsightly dirt and grime tends to accumulate in the gap between the bezel and the front sheet. Moreover, the bezel prevents achieving an aesthetically pleasing planar front surface for the touchscreen display.
In order to highlight the advantages of the present invention, a typical prior art touchscreen and display assembly will first be described, followed by a detailed description of a preferred embodiment of the present invention. In the descriptions, items performing similar functions in both the prior art and in the invention are designated with the same numeral, but with the numeral for the prior art item being followed by a prime symbol (′). It should be noted that construction of items with the same numeral may be quite different and that use of the same numerals is made only for the sake of clarity of the description.
FIG. 3A is a schematic partial plan view of a prior art touchscreen display with its front casing removed. A front sheet 10′ is the surface presented to the user for touching and for thereby activating the functions of the touchscreen. A gasket 20′, made of any suitable soft material such as an elastomer, is placed on top of and on the outer perimeter of front sheet 10′.
FIG. 3B is a section view in the plane C-C of FIG. 3A, and FIG. 3C is an expanded section view of Detail D of FIG. 3B. FIGS. 3B and 3C show an integrated display screen 18′, and also include views of a front casing 16′, which is not shown in FIG. 3A. It can be seen that gasket 20′ is in contact both with front sheet 10′ and a bezel 24′, with bezel 24′ being defined as that part of casing 16′ which overlaps the outer perimeter of front sheet 10′. Gasket 20′, being of soft material, and being compressed between front sheet 10′ and bezel 24′, forms a seal designed to prevent ingress of dirt, particles and corrosive vapors.
It should be noted that the need for contact between gasket 20′ and front sheet 10′ is a significant disadvantage of the prior art design because there is a danger that proper operation of the touchscreen may be degraded or altogether prevented. In particular, any direct contact or bonding between front sheet 10′ and bezel 24′ will restrict free movement of front sheet 10′ and must be avoided. It is therefore difficult both to ensure a reliable seal and to avoid inadvertent interruption of touchscreen operation.
A further disadvantage is the need to use bezel 24′ to mount and seal the touchscreen display, making it impossible to achieve a more aesthetically pleasing smooth front face. In addition, the gap between bezel 24′ and front sheet 10′ tends to collect unsightly particles of dirt 22′ as shown in FIG. 3C.
The particular design and location of gasket 20′ in the prior art has been driven by the need to allow access into the touchscreen display enclosure (not shown) of a touchscreen cable 6′ and an optional vent aperture 14′. Touchscreen cable 6′ may be any type of flat cable, but is usually a flexible printed circuit (FPC) cable whose function is to make the necessary electrical connections for operation of the touchscreen. Touchscreen cable 6′ is connected at one end to a number of touchscreen electrodes 12′ and passes through a hole 26′ in front casing 16′. Touchscreen electrodes 12′ are thin layers of conductive material which are printed or deposited on the inner surfaces of a glass panel 8′ and front sheet 10′. In a typical example, there may be two electrodes 12′ on glass panel 8′ and two electrodes 12′ on front sheet 10′, and cable 6′ has four conductors, with one conductor being connected to each electrode. An adhesive sheet 2′ is placed between the touchscreen electrodes 12′, and serves to seal an airgap 9′ between top sheet 10′ and glass panel 8′. Vent aperture 14′ is optionally provided to allow equalization of air pressure between touchscreen airgap 9′ and the external environment if required. Vent aperture 14′ consists of a gap in adhesive sheet 2′ (shown by dotted lines in FIG. 3A), and a corresponding hole (not shown) in front casing 16′. Due to the need for access into the touchscreen display enclosure of touchscreen cable 6′ and vent aperture 14′, while still providing a seal between the touchscreen and front casing 16′, gasket 20′ has been designed to lie in a plane exterior to the plane of touchscreen cable 6′, adhesive sheet 2′ and vent aperture 14′. This is most clearly shown in FIG. 3C.